Solid-state image sensors, in particular for example CMOS image sensors are known in the art. Many modern CMOS sensors are overlaid with a colour filter array (CFA) for obtaining three (or more) colour components, e.g. red, green and blue. Commercially the most commonly used CFA configuration is the so called Bayer configuration. It has a pattern of four pixels arranged in a square, two opposite corners being taken by green pixels, the other two corners being taken by a red pixel and a blue pixel, as shown in FIG. 1 (left). Although this configuration takes into account that the human eye is more sensitive to green light than to red or blue light, such image sensors are not perfect, and they do not reproduce exactly the same colours as perceived by the human eye. Because the human eye and the silicon devices use completely different methods to capture information, it is not surprising that the silicon devices do deviate from the human visual system.
Besides differences with the HVS (human visual system), silicon devices also suffer from other artefacts, such as for example cross-talk. Several forms of cross-talk are known in the art, such as spectral cross-talk, optical cross-talk and electrical cross-talk. These provide additional artefacts, which need to be compensated for.
Another aspect of digital cameras is auto-focus. At present there are two major systems for autofocus operation in solid-state cameras:
1) Contrast detection: simple, without extra components in the camera, but slow, consuming quite some power and relative measurements are being taken. This system is used in cheap cameras (e.g. mobile imaging),
2) Phase detection: fast and accurate, but requires extra lenslets and a dedicated sensor, absolute measurements are being taken.
In today's mirror-less cameras, there is a clear need for a high-quality auto-focus system, but without a mirror, so that the classical phase detection method is no longer possible. For that reason, the sensors of these modern cameras are provided with drop-in pixels that can function as auto-focus pixels.
US20090167927 describes a digital camera with such an image sensor having image pixels and non-image pixels. The non-image pixels are different from the surrounding image pixels, and are intended for providing information for auto-focus. They do not contribute to the normal shooting of an image, i.e. they do not provide actual image data. When taking an image with such a sensor, the drop-in pixels are treated as defect pixels, and the image is corrected accordingly. US20090167927 discloses an algorithm for correcting such drop-in pixels, by estimating image information based on image data obtained from surrounding image pixels. However, this algorithm does not correct for cross-talk.